1. Field of the Invention
The present invention relates to an image processing apparatus for half-tone processing input image data by using a threshold value matrix consisting of a plurality of threshold values and, more particularly, to an image processing apparatus which can accurately reproduce input images having different resolutions.
2. Description of the Related Background Art
Hitherto, a dither method has been known as a technique for half-tone processing input image data.
According to the dither method, by binarizing a plurality of input image data by using a threshold value (dither) matrix consisting of a plurality of threshold values, a half-tone image can be reproduced.
However, hitherto, there is a case where even if the resolution of the input image data was changed, the half-tone processing is executed by using the same dither matrix, so that the input image cannot be accurately reproduced.
FIG. 4 shows a spiral-type systematic dither matrix as an example of the dither matrix.
Each of a plurality of input image data is compared with each of the threshold values in the dither matrix of FIG. 4. When the input image data is larger than the threshold value, a signal indicative of black is output. When the input image data is smaller than the threshold value, a signal indicative of white is output. Due to this, the half-tone processing can be performed.
For instance, when image data having a resolution of 400 ppi (pel per inch).times.400 ppi and a uniform concentration of a concentration level of 17 is input and binarized by using the dither matrix of FIG. 4, a processing result shown in FIG. 6 is obtained.
On the other hand, when image data having a resolution of 200 ppi.times.200 ppi and a uniform concentration of a concentration level of 17 is input and binarized by using the same dither matrix (FIG. 4) as that in the above case of the resolution of 400 ppi, a processing result shown in FIG. 13 is obtained.
As mentioned above, in the case where a plurality of input image data of different resolutions were half-tone processed by using the same dither matrix, a pitch of the output image changes by the resolution. There is a drawback such that in spite of the fact that only the resolutions are different, the picture qualities of the images after completion of the processings largely change for the input images of the same picture quality.